The present invention relates to an apparatus and methods for electrospinning polymer fibers and membranes.
Electrospinning is an atomization process of a conducting fluid which exploits the interactions between an electrostatic field and the conducting fluid. When an external electrostatic field is applied to a conducting fluid (e.g., a semi-dilute polymer solution or a polymer melt), a suspended conical droplet is formed, whereby the surface tension of the droplet is in equilibrium with the electric field. Electrostatic atomization occurs when the electrostatic field is strong enough to overcome the surface tension of the liquid. The liquid droplet then becomes unstable and a tiny jet is ejected from the surface of the droplet. As it reaches a grounded target, the material can be collected as an interconnected web containing relatively fine, i.e. small diameter, fibers. The resulting films (or membranes) from these small diameter fibers have very large surface area to volume ratios and small pore sizes. However, no practical industrial process has been implemented for electrospinning membranes containing a high percentage of small, e.g., nanosize, fibers. This is because with the production of small fibers, such as nanosize fibers, the total yield of the process is very low and a scale-up process, which maintains the performance characteristics of the films (or membranes), cannot be easily achieved.
U.S. Pat. No. 4,323,525 is directed to a process for the production of tubular products by electrostatically spinning a liquid containing a fiber-forming material. The process involves introducing the liquid into an electric field through a nozzle, under conditions to produce fibers of the fiber-forming material, which tend to be drawn to a charged collector, and collecting the fibers on a charged tubular collector which rotates about its longitudinal axis, to form the fibrous tubular product. It is also disclosed that several nozzles can be used to increase the rate of fiber production. However, there is no suggestion or teaching of how to control the physical characteristics of the tubular product, other than by controlling the charge and rotation speed of the tubular collector. For example, there is no teaching or suggestion of controlling jet formation, jet acceleration or fiber collection for individual jets. It is further noted that the spinning process of the ""525 patent is used to fabricate tubular products having a homogenous fiber matrix across the wall thickness.
U.S. Pat. No. 4,689,186 is directed to a process for the production of polyurethane tubular products by electrostatically spinning a fiber-forming liquid containing the polyurethane. It is disclosed that auxiliary electrodes can be placed around the collector to help facilitate collection of the fibers. It is disclosed that the auxiliary electrodes can be arranged to facilitate separation or to prevent adhesion of the formed fibers. There is no teaching or suggestion of independently controlling jet formation, jet acceleration and fiber collection. It is also noted that the spinning process of the ""186 patent is used to fabricate tubular products having a homogenous fiber matrix across the wall thickness.
The above mentioned references do not address the problems associated with producing membranes or other articles on an industrial scale, without adversely affecting the performance characteristics of the resulting products.
Thus, there is a need for improved electrospinning methods for producing fibers and membranes on an industrial scale which do not have the above-mentioned disadvantages.
According to the present invention, it has now been found that polymeric fibers can be produced by an elecrospinning process having improved control over fiber formation and transportation. In addition, membranes can be produced by electrospinning with the apparatus and according to the methods of the present invention on an industrial scale without the above-mentioned disadvantages.
In one aspect, the invention relates to a method for electrospinning a polymer fiber from a conducting fluid containing a polymer in the presence of a first electric field established between a conducting fluid introduction device and a ground source, which includes modifying the first electric field with a second electric field to form a jet stream of the conducting fluid. The conducting fluid introduction device is preferably a spinneret.
The second electric field can be established by imposing at least one field modifying electrode on the first electrostatic field. The field modifying electrode can be a plate electrode positioned between the conducting fluid introduction device and the ground source.
Preferably, the method includes feeding the conducting fluid to the conducting fluid introduction device at a controlled rate. The rate can be controlled by maintaining the conducting fluid at a constant pressure or constant flow rate.
In one embodiment, the method also involves controlling the electrical field strength at the spinneret tip by adjusting the electric charge on the field modifying electrode to provide a controlled diameter fiber.
In another embodiment, the method includes imposing a plurality of electrical field modifying electrodes to provide a controlled distribution of electrostatic potential between the spinneret and the ground source.
In another aspect, the invention relates to a method for electrospinning a polymer fiber from a conducting fluid containing a polymer in the presence of an electric field established between a spinneret and a ground source, which includes:
a) forming an electrospinning jet stream of the conducting fluid; and
b) electrically controlling the flow characteristics of the jet stream.
The flow characteristics of the jet stream can be electrically controlled by at least one electrode. The flow characteristics of the jet stream can also be electrically controlled by at least one pair of electrostatic quadrupole lenses. Preferably, the flow characteristics of the jet stream are electrically controlled by a plurality of pairs of electrostatic quadropole lenses and, more preferably, by also using an alternating gradient technique.
In one embodiment, the method involves electrically controlling the flow characteristics of the jet stream to provide a controlled pattern over a desired target area. The controlled pattern can be provided by applying a waveform to the potential on at least one pair of electrostatic quadropole lenses.
In yet another aspect, the invention relates to a method for forming a controlled-dimension and controlled-morphology membrane by electrospinning a plurality of polymer fibers from conducting fluid containing a polymer in the presence of an electric field established between a solution introduction device and a ground source, in which the method includes:
a) forming a plurality of electrospinning jet streams of the conducting fluid; and
b) independently controlling the flow characteristics of at least one of the jet streams.
Preferably, the flow characteristics of at least one of the jet streams are electrically controlled by at least one scanning electrode, more preferably, by at least one pair of scanning electrodes.
In one embodiment, the solution introduction device consists of a plurality of electrospinning spinnerets. Preferably, each spinneret produces an individual jet stream of the conducting fluid and, more preferably, the flow characteristics of each individual jet stream can be independently controlled.
Preferably, each spinneret has at least one scanning electrode for electrically controlling the flow characteristics of the individual jet stream. More preferably, each spinneret has two pairs of scanning electrodes for electrically controlling the flow characteristics of the individual jet stream.
It is contemplated that at least two spinnerets can deliver different solutions, wherein different solutions refers to different concentrations of polymer, different polymers, different polymer blends, different additives and/or different solvents.
In another aspect the invention is directed to an electrospinning apparatus for forming a membrane, which includes:
a conducting fluid introduction device for providing a quantity of conducting fluid containing a polymer, the conducting fluid introduction device containing a plurality of electrospinning spinnerets for delivering the conducting fluid, the spinnerets being electrically charged at a first potential;
a ground member positioned adjacent to the spinnerets and electrically charged at a second potential different from the first potential, thereby establishing an electric field between the spinnerets and the ground member;
a support member disposed between the spinnerets and the ground member and movable to receive fibers formed from the conducting fluid; and
means for controlling the flow characteristics of conducting fluid from at least one spinneret independently from the flow characteristics of conducting fluid from another spinneret.
Preferably, the means for independently controlling the flow characteristics includes at least one electrode disposed adjacent each spinneret, each electrode being charged at a potential different from and separate from the first potential.
Preferably, each spinneret has two pairs of scanning electrodes for electrically separately controlling the flow characteristics of conducting fluid from the spinneret.
The means for independently controlling the flow characteristics can include a means for individually electrically turning on and off a respective spinneret. Preferably, the means for individually electrically turning on and off a respective spinneret contains at least one scanning electrode associated with each spinneret.
The means for independently controlling the flow characteristics can also contain a means for applying an alternating gradient to the conducting fluid delivered from the spinnerets. Preferably, the means for applying said alternating gradient includes a plurality of pairs of electrostatic quadropole lenses.
In one embodiment, the electrospinning apparatus includes a probe associated with at least one spinneret, the probe being disposed between the electrode and the ground member, the probe being electrically charged at a potential different from the spinneret and the electrode.
The electrospinning apparatus will preferably contain a pump for supplying conducting fluid to the conducting fluid introduction device at a predetermined pressure. The pump can also be adapted to control the supply rate of conductive fluid at a constant flow rate or at a constant pressure.
The electrospinning apparatus will preferably include a pump system for supplying different conducting fluids to at least two individual spinnerets.
In one embodiment, the conducting fluid introduction device contains a slit-die defining the plurality of spinnerets. The adjacent spinnerets can be interconnected by slits. In such an embodiment, the spinnerets can be defined by openings in the slit-die and the slits interconnecting the spinnerets are of configurations smaller than the openings. The apparatus can also contain a plurality of scanning electrodes disposed adjacent to each of the spinnerets.
In another embodiment, the solution introduction device includes a matrix defining the plurality of spinnerets, the spinnerets being disposed in the matrix in electrical isolation from each other. At least two individual spinnerets can be electrically charged to a different potential. The solution introduction device can also contain a plurality of individual electrodes in which at least one individual electrode is disposed adjacent to each individual spinneret. At least two individual electrodes can be electrically charged to a different potential.
In yet another aspect, the invention is directed to an apparatus for forming a membrane by electrospinning a plurality of polymer fibers from a conducting fluid which contains a polymer in the presence of an electric field between a conducting fluid introduction device and a ground source, in which the apparatus contains an improved conducting fluid introduction device which includes a plurality of spinnerets, each for independently delivering a controlled quantity of conducting fluid at a controlled pressure or flow rate, the spinnerets being charged at an electric potential and being disposed relative to each other to normally interfere with the electric field produced by adjacent spinnerets, each of the spinnerets having a tip at which conducting fluid exits configured to have an electrostatic field strength at each tip stronger than the liquid surface tension at each of the tips.
Each of the tips can be configured by having a tip with a selected geometric profile, a selected spatial relationship relative to other spinneret tips or a combination of both.
The apparatus containing the improved conducting fluid introduction device can also include an electrode associated with each spinneret configured to produce an electrical potential to at least partially screen electric field interference from adjacent spinnerets.
The apparatus containing the improved conducting fluid introduction device can also include a means for at least partially shielding a spinneret from electric field interference from adjacent spinnerets. The means for shielding can be a physical barrier disposed between adjacent spinnerets. The barrier will preferably have a conical shape.
The present invention provides an apparatus and methods for producing fibers and membranes by electrospinning with improved control over fiber formation and transportation. It also provides an apparatus and methods for producing membranes containing nanosize fibers on an industrial scale, without the above-mentioned disadvantages.